The present invention is directed to recapture and/or recycle of perfluorocompounds (PFCs). In particular, the present invention is directed to recovery of PFCs used in semiconductor manufacturing processes for use in chemical vapor deposition (CVD) chamber cleaning.
PFC gas mixtures or compounds (e.g., C2F6 or CF4) are used in semiconductor manufacturing processes as a convenient source of fluorine for the plasma etching of silicon-based materials (e.g., SiO2, SiNx, poly-Si). Due to strong infrared absorbances and long atmospheric lifetimes, however, PFCs are suspected of contributing to global warming. Responding to this concern, the global semiconductor industry has voluntarily agreed to minimize its PFC emissions. Reduction targets for the U.S., Europe and Japan are 10% of 1995 levels by the year 2010. A strategy for achieving these emissions reductions is to capture the PFCs from the process effluent. For example, a membrane-based PFC recovery system has been demonstrated in M. Foder, R. Wimmer, J. Yang, and T. McCay, “Recovery of Perfluorocompounds (PFCs) from Semiconductor Manufacturing Processes Using a Membrane-Based System,”, ECS Proceedings, 99-8, 60 (1999).
Additionally, typically, CVD chambers are cleaned using a C2F6/O2-based RF plasma process as follows:C2F6+O2+SiO2→C2F6+CF4+O2+SiF4+COF2+CO2
Here, PFC recovery is necessary because the influent C2F6 is not completely utilized, i.e., the influent stream as well as the effluent stream contains C2F6. Also, the plasma process generates some CF4. Since the PFC recovery system removes all of the acid gases (e.g., SiF4, COF2 and HF) and is designed to separate PFCs from atmospherics gases (e.g., O2, N2, CO2), the captured material is a mixture of both C2F6 and CF4. The generation of CF4 during the CVD chamber clean process makes using the recovered PFC gas more difficult.
Attempts to reuse the recovered PFCs have focused on separating the mixture into its components (C2F6 and CF4) followed by further purification. See, e.g., U.S. Pat. No. 5,502,969 (Jin et al.) directed to a cryogenic rectification system for fluorine compound recovery. Recovered C2F6 can then be used in existing C2F6-based chamber clean processes. The goal is for the recovered components to meet the purity specifications of the virgin product. All impurities (including other PFCs) must only be present at trace levels. Typically, impurity levels required are less than 5 parts per million (ppm) except for N2 which is around 250 ppm and O2 which is around 100 ppm.
A process has also been developed whereby the PFC mixture is separated into its components via cryogenic distillation. The concentration of impurities in recovered C2F6 can indeed be reduced to the levels of virgin product. The shortcoming of this approach is economic. The cost of separating the PFC mixture and subsequent purification is higher than the cost of manufacturing the virgin C2F6 material. Additionally, distillation could be difficult if the recovered PFC's contain unsaturated fluorocarbons, such as CHF3, C2HF5, etc. that may form azeotropes with C2F6, resulting in the loss of valuable product. Furthermore, the presence of NF3 having a close boiling point to CF4 makes their separation by distillation practically impossible. At any rate, the recovered C2F6 and CF4 gases will not be as pure as the virgin material and will cost more to produce.
As taught in U.S. application Ser. No. 09/542,995, filed Apr. 4, 2000, “Reclamation and Separation of Perfluorocarbons Using Condensation.”, by W. T. McDermott, R. C. Ockovic, A. Schwarz, and R. Agrawal, cold trapping of PFC effluents, rather than membrane separation, may provide an economical feedstock for mixed PFC chamber clean processes. Separation of PFCs by condensation may not completely recover C2F6 and CF4, i.e., recovered effluent is not separated into single components with all impurities present at trace levels. This is, however, more economical than distillation and can be used as mixed PFC source gas for CVD chamber cleaning.
In Andrew D. Johnson et al., “Minimizing PFC Emissions from Existing PECVD Tools: Optimization of the Chamber Clean Process of Record, Semicon West 2000, minimization of PFC emissions from cleaning PECVD chambers is described where the chamber clean process is optimized. Such optimization could occur by adjusting C2F6 flowrate, adjusting O2:C2F6 ratio and/or adjusting pressure.
As indicated in M. A. Sobelewski, J. G. Langan, and B. S. Felker, “Electrical Optimization of Plasma-Enhanced Chemical Vapor Deposition Chamber Cleaning Plasmas,” J. Vac. Sci. Technol., B16, 173 (1998), the use of C2F6-based and CF4-based plasmas to clean CVD equipment is widespread. Typically, O2 is added to either C2F6 or CF4 to inhibit polymer deposition and enhance the etch rate. Mixed PFCs are not expected to provide for improvements in cleaning performance.
Finally, in Nakata, Kubota, Kaji, Yoda and Okumura, “Reduction of PFC Emissions by Gas Recirculation Cleaning in Plasma CVD,” IEEE, 2001, a process is disclosed that is directed to a method for cleaning a CVD chamber using gas recirculation. Unused gas contained in the exhaust gas is returned to the chamber by a pump. No virgin C2F6 or CF4 is added.
It is principally desired to provide viable reuse of PFCs recovered from semiconductor tool effluents.
It is further desired to provide viable reuse of PFCs recovered from effluents that uses a mixed PFC source gas of C2F6 and CF4 to clean, for example, CVD chambers.
It is still further desired to provide viable reuse of PFCs recovered from semiconductor tool effluents where it is not necessary to separate and purify recovered PFC effluents.